The Lunar Surface and Late Heavy Bombardment Concept

Abstract

As much as half of lunar surface rocks may have originated between 4.4 and 3.9 billion years and thus observations of, and samples from, Moon could attest to conditions then extant in the inner solar system. The concept of a lunar cataclysm at ~3.9 Ga grew from seemingly contradictory observations of elemental fractionation in lunar highland rocks. U–Pb—and some Rb–Sr—data suggested recrystallization occurred between about 4.0 and 3.8 Ga. The Late Heavy Bombardment (LHB) concept that emerged appeared supported by ~3.9 Ga 40Ar/39Ar “plateau ages” of lunar impact melt rocks, although no similar spike in ages was seen in the likely more globally distributed lunar meteorites. While the 40Ar/39Ar step-heating method can reveal intragrain isotope variations, this capability has several method-specific requirements that, if not met, preclude thermochronologic interpretations. Three such issues effectively rule out the use of virtually all lunar 40Ar/39Ar data as support for the LHB hypothesis: (1) the “plateau age” approach used is an aphysical concept for the thermally disturbed samples typical of most lunar impact melt rocks, (2) laboratory artifacts destroy preserved diffusion information, or create false apparent age gradients; and (3) obtaining meaningful thermal history information from extraterrestrial samples that have differing activation energies for Ar diffusion in their K-bearing phases requires a different laboratory protocol than was used on lunar rocks. Possibly due to these issues, no case in which multiple chronometric techniques have yielded intrasample concordancy of a lunar melt rock has yet been documented. Advancements in mass spectrometry now permit 40Ar/39Ar and U–Pb dating to be undertaken on small (10 s-of-μm diameter) in situ spots on glasses and accessory minerals in lunar rocks. This approach has the potential to transcend the analytical challenge posed by the continuous impact reworking of the lunar regolith that produces fine-scale polygenetic breccias of multiple age and origins. The longstanding assumption that lunar melt rocks originated from discrete, basin-forming events is obviated by lunar imaging that show impact melts formed in small highland craters and clusters of ‘light plains’ deposits radiating outward >2000 km from large impact basins. The latter underscores how poorly the spatial relationships between large basins and their surrounding deposits were understood when impact chronologies were developed in the 1970s. The assumption that a specific lunar melt rock from a given landing site is representative of one of the basin-forming impacts is deeply flawed. Establishing a reliable, quantitative planetary impact chronology requires that all analyzed rocks be equally suitable for the application of specific chronometers. This may not be possible given the large contrasts in incompatible trace element distributions across the lunar surface (e.g., Procellarum KREEP terrane, South Pole Aiken basin). A conservative view of the lunar chronological record is that the large nearside basins are older than 3.82 Ga but these data are consistent with most of them being older than 3.92 Ga and possibly older than 4.35 Ga.